The output power of an electric machine is critically dependant on accurate synchronisation of phase excitation and rotor position. Accordingly, the electric machine typically includes a position sensor for determining the position of the rotor.
Any variance in the duty cycle of the position sensor will result in a phase difference between the detected position and the actual position of the rotor. Consequently, phase excitation may not be perfectly synchronised with the rotor position over an electrical half cycle, and thus the power and efficiency of the motor may be reduced.
For many electric machines, any variance in duty cycle is not normally regarded as a problem. This may be because the circumference of the rotor is relatively large and thus any variance in duty cycle results in a negligible phase difference between the detected and actual positions of the rotor. Alternatively, the output power and/or the efficiency of the electric machine are not critical and thus any power losses that arise from duty-cycle variance are deemed acceptable. However, for electric machines that are relatively small and/or where relatively high efficiency is required, variance in the duty cycle can present a significant problem.